Very specific demands have to be met by high temperature applications such as piston rings that are intended for use in for instance marine diesel engines, particularly as concerns strength, anti-corrosive properties, wear resistance, ductility and material resilience.
When used in a diesel engine the piston ring is arranged to abut on the one hand against an associated piston groove, on the other against an engine cylinder-bore. Consequently, the ring should be wear-resistant, particularly at the interface towards the cylinder bore, where high friction is generated when the engine is in operation.
The piston ring should therefore also possess an inherent tension or resilience whereby the piston ring will constantly be forced outwards, into abutment against the cylinder bore. In addition, upon each explosive stroke of the engine, the piston ring is urged with considerable force radially outwards, into abutment against the cylinder bore, with consequential increase of stress. Due to a high working temperature in engines and especially due to the impact of produced heat, from contact between piston rings and cylinder liner during the process, many materials loose some of their yield strength and show softening.
Today, piston rings are generally manufactured from a cast-iron blank, which substantially meets the requirements imposed on the material as regards strength and resilience but generally not, especially when heated, on wear resistance on the surface thereof that faces the cylinder bore. Cast iron does not possess the required thermal stability at high temperature. A cast-iron piston ring blank therefore usually is provided with a wear-resistant layer on the surfaces most exposed to wear.
However, difficulties do arise in achieving a sufficiently strong bond between the material of the blank and the material of the wear layer, which causes problems, because of the risk that the material of the wear layer be torn away from the material of the blank. When this happens, the comparatively soft material of the blank-material surface is exposed to wear in the area of contact against the cylinder bore, with resulting considerable shortening of the life of the piston ring.
Another issue is that the coating gradually wears away, even if the bond between the surfaces is comparatively strong. The wear on the piston ring progresses slowly as long as the wear layer is intact but very rapidly, once that layer has been damaged. As a result, it may be difficult to determine in time when a piston ring change should be made.
Thermal spraying is conveniently used to apply coatings on piston rings. Generally, one issue with using thermal spraying for applying a coating is that the resulting coating comprises a fraction of loose particles. These loose particles are increasing the risk for “three-body-wearing” between for example the coated piston ring and cylinder liner. Three-body-wearing is often an initiator of the process of gradual wearing indicated in the above.
In operation, especially some contact areas between the piston ring and cylinder bore or liner material are exposed to high temperatures, to considerable temperature differences, and to the effects of a highly corrosive environment. In order to withstand the effects of these stress-inducing causes, the piston ring therefore also must exhibit considerable ductility, and thermal stability in addition to the before mentioned wear resistance. By ductility is to be understood herein the maximum possible deformation of the material before cracking begin.
It is thus desirable to achieve a coating for piston rings, which is resistant to wear, thermal chock, corrosion and oxidation. It is known various methods to post-heat or sinter a substrate after a coating has been applied in order to provide a strong bond between coating and substrate. U.S. Pat. No. 5,268,045 provides an example of such prior art coating method, wherein a work piece to be coated is electrochemically cleaned, thermal spray coated with a metal or metals in order to provide an overlay coating and post heat treated at an elevated temperature, typically for about two hours, to diffuse said metal or metals into the surface of the work piece.
During such a process the coating will reach its melting temperature and there is a risk that also the underlying substrate is affected in such a way that stress is induced in the substrate. This is especially an issue for piston ring blanks. Other issues related to prior art techniques will be described more in detail in the document. Hence it is desirable to find a method for applying a coating on a piston ring with minimized induced stress in the piston ring blank and also reducing the risk of loose particles within the coating. At present no method is known of applying a coating for piston rings to overcome the above issues.